In a manufacturing process of an electronic device such as a semiconductor device, etching is performed to form a mask on an etching target layer and transcribe a pattern of the mask to the etching target layer. In general, a resist mask is used as the mask. The resist mask is formed by photolithography. Thus, a critical dimension of the pattern formed in the etching target layer is affected by a resolution limit of the resist mask formed by the photolithography.
With a recent trend of high integration of electronic devices, however, it is required to form a pattern having a dimension smaller than the resolution limit of the resist mask. In this regard, as described in Patent Document 1, there has been proposed a technique of reducing a width of an opening of the resist mask by depositing a silicon oxide film on the resist mask.
To be specific, in the technique disclosed in Patent Document 1, the silicon oxide film is formed on the resist mask by an atomic layer deposition (ALD) method. More specifically, a source gas containing organic silicon and activated oxygen species are alternately supplied into a processing vessel in which a processing target object is accommodated. An aminosilane gas is used as the source gas.
Patent Document 1: Japanese Patent Laid-open Publication No. 2011-082560
In the technique of reducing the opening width of the mask by using the silicon oxide film, which is formed on a surface of the processing target object including a surface of the mask by using the film forming method such as the ALD method, non-uniformity in thicknesses of the silicon oxide film at individual positions on the surface of the processing target object needs to be reduced. That is, when forming the silicon oxide film, the silicon oxide film needs to have high in-plane uniformity on the surface of the processing target object and, also, to have conformal coatability. Here, the term “conformal coatability” implies that a difference between a thickness of the silicon oxide film on a top surface of the mask, a thickness (width) of the silicon oxide film on a side surface of the mask confining the opening and a thickness of the silicon oxide film on a bottom surface of the opening is small. If, however, an aspect ratio of the opening of the mask is increased, the thickness of the silicon oxide film on the side surface of the mask confining the opening and the thickness of the silicon oxide film on the bottom surface of the opening are reduced as compared to the thickness of the silicon oxide film formed on the top surface of the mask.
Accordingly, it is required to reduce the non-uniformity in the thickness of the silicon oxide film formed on the processing target object even when the aspect ratio of the opening of the mask is increased.